The invention relates to a hydrocarbon treating process referred to as sweetening. In this process, mercaptans present in a liquid hydrocarbon stream such as naphtha or kerosene are oxidized in the presence of an aqueous alkaline solution to disulfide compounds which remain in the hydrocarbon stream. The sweetening of sour petroleum fractions is a well developed commercial process which is employed in almost all petroleum refineries. In this process, mercaptans present in the feed hydrocarbon stream are converted to disulfide compounds which remain in the hydrocarbon stream. Sweetening processes, therefore, do not remove sulfur from the hydrocarbon feed stream but convert it to an acceptable form. The sweetening process involves the admixture of an oxygen supply stream, typically air, to the hydrocarbon stream to supply the required oxygen. The admixture of hydrocarbon and air contact an oxidation catalyst in an aqueous alkaline environment. The oxidation catalyst may be impregnated on a solid composite or may be dispersed or dissolved in the aqueous alkaline solution. A commonly employed oxidation catalyst comprises a metal phthalocyanine compound impregnated on activated charcoal. A suitable catalyst is described in U.S. Pat. No. 4,049,572.
Unfortunately, the aqueous alkaline solution is neutralized over time by acidic components of the hydrocarbon stream, requiring its continued replacement and replenishment. This is especially true for certain feedstocks, such as kerosene, which typically have a significant content of naphthenic acids. Naphthenic acids are carboxylic acids found in petroleum and various petroleum fractions during refining. See Kirk-Othmer, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY (3d ed. 1981) Vol. 15, 749–53. Naphthenic acids are predominantly monocarboxylic acids having one or more cycloaliphatic groups alkylated in various positions with short chain aliphatic groups and containing a polyalkylene chain terminating in the carboxylic acid function. Although cyclopentane rings are the predominant cycloaliphatic ring structure, other cycloaliphatics rings, such as cyclohexanes, also may be present in appreciable quantities. The naphthenic acid content of feedstocks such as kerosene engenders further complications arising from the limited solubility of alkali metal naphthenates in concentrated alkali. Insoluble alkali metal naphthenates tend to plug beds of alkaline wetted oxidation catalyst. To avoid this, kerosene and kerosene range feedstocks undergo a caustic prewash to remove naphthenic acids prior to entry of the feedstock to the fixed bed. The solubility of the alkali metal naphthenates are such that their efficient extraction from kerosene range feedstocks into aqueous media requires prewash by a dilute caustic, with a concentration usually under 3 wt-%.
Two mercaptan oxidation sweetening processes employing fixed catalyst beds are described in D. L. Holbrook, HANDBOOK OF PETROLEUM REFINING PROCESSES, 11.31–.33 (Robert A. Meyers 2d ed. 1996). The first described process is fixed-bed sweetening which may be employed for heavier feedstocks having endpoints above 120° C. (248° F.) such as kerosene or jet fuel. The heavier feedstocks are only slightly soluble in caustic and hence more difficult to sweeten. Moreover, the heavier feedstocks have a density that is closer to that of the typically caustic alkaline solution which makes separation of sweetened hydrocarbon from the alkaline solution more difficult. Fixed-bed sweetening employs a reactor that contains a bed of activated charcoal impregnated with mercaptan oxidation catalyst and periodically wetted with aqueous caustic solution. Air is injected into the feed before entry into the reactor. The catalyst oxidizes the mercaptans in the feed to disulfides. A single outlet conducts the product hydrocarbon stream and aqueous caustic to a settler in which the hydrocarbon phase and aqueous caustic phase are separated. Caustic is withdrawn from the bottom of the settler periodically and circulated over the catalyst bed to maintain alkalinity. The catalyst bed is rinsed with aqueous caustic about once a day for up to half an hour to clean the pores of the catalyst and to alkalinize the catalyst bed. Greater amounts of aqueous caustic must be circulated over the catalyst bed for kerosene applications because more aqueous caustic is needed to solubilize the less soluble kerosene range hydrocarbons and to clean the pores of the catalyst in which the larger kerosene molecules tend to become lodged. The large volume of caustic is conducted to the settling tank to be pumped back to the reactor during the next rinse.
The second described process uses less equipment to sweeten lighter feeds such as catalytically cracked naphthas and light virgin naphthas. In this process, relatively weak aqueous caustic is continuously injected into a hydrocarbon feed previously freed of hydrogen disulfide. The caustic-hydrocarbon mixture is then mixed with air and delivered to a fixed catalyst bed in the reactor. The sweetened naphtha is removed near the bottom of the reactor above the hydrocarbon-caustic interface and caustic drains into a drain interface pot. The drain interface pot separates hydrocarbon from caustic and sends the former back to the reactor while the latter is treated and/or disposed. A similar process for fixed-bed sweetening of gasoline is shown at page 124 of the April, 1982 issue of HYDROCARBON PROCESSING. A very small amount of the aqueous solution is continuously charged to the reactor vessel. The aqueous solution is then withdrawn from the bottom of the reactor vessel. However, the article indicates that a larger amount of more concentrated caustic must be intermittently recirculated over the catalyst requiring a settling tank to treat heavier hydrocarbon feeds, such as kerosene.
Continuous injection of alkaline solution into the mercaptan oxidation reactor with separation between the alkaline solution and the sweetened hydrocarbon in the same reactor has been proposed for treating heavier feeds. U.S. Pat. No. 4,481,106 discloses an apparatus for fixed bed sweetening which separates caustic and hydrocarbon phases in an annular separation zone separated by cylindrical screen from an inner catalyst bed. However, there has been concern that the volume of continuously injected alkaline solution necessary to sufficiently alkalinize a kerosene range hydrocarbon feed may be too great to effect separation in the reactor vessel.
Other mercaptan oxidation reactor vessels that include a discrete separation section have been proposed. U.S. Pat. No. 4,019,869 illustrates a reactor vessel that includes a catalyst bed resting on a horizontal support. Sweetened hydrocarbon is separated from aqueous caustic in the catalyst bed and delivered to a discrete subjacent portion of the reactor vessel. The aqueous alkaline-hydrocarbon interface is developed in subjacent portion of the reactor vessel to effect a second separation. U.S. Pat. No. 5,961,819 discloses a reactor vessel including a first downflow reaction zone over a fiber bundle that ends in a separator zone and a second upflow reaction zone over a mercaptan oxidation catalyst.
Sweetened kerosene reactor effluent from the settling tank is typically delivered to an alkali removing unit such as a salt filter or water wash vessel and then to a product tank. On the other hand, sweetened kerosene product must meet stricter specifications to be used as a jet fuel. To meet jet fuel specifications, oil-soluble surfactants must be removed from the sweetened kerosene. Such surfactants may be removed by running the sweetened kerosene through a residual surfactant removal device such as a clay filter. However, clay filters are sensitive to alkali. Hence, alkali remaining in sweetened kerosene must be removed such as by a water wash. Additionally, water must be removed from the kerosene in a residual water-removing device such as a salt filter before it is filtered in the clay filter. The amount of water in the kerosene effluent from the water wash is proportional to the salt that is consumed in the salt filter. Conventionally, water was batch replaced in the water wash vessel when the alkaline concentration in the water reached a high level necessitating greater labor and decreased separation efficiency during replacement. Additionally, because the water is mixed with the sweetened hydrocarbon in the water wash vessel, a greater proportion of water volume in the water wash vessel is necessary, thus diminishing the residence time of the hydrocarbon in the water wash vessel.
An object of the present invention is to provide a mercaptan oxidation apparatus and process for heavy hydrocarbon feed that includes a reaction section and a separation section in the same reactor vessel.
Another object of the present invention is to provide a mercaptan oxidation apparatus and process that obviates a settling tank after the reactor vessel.